1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly to a liquid crystal display device in which a light loss of the liquid crystal display device is reduced in a transmission mode and a bi-directional display is provided.
2. Description of the Related Art
In these days, electronic display devices become more important for communicating and processing various information. Also, various types of electronic display devices are widely used in different industrial fields.
Generally, an electronic display device visually provides a variety of information to a user. In other words, an electrical information signal output from electronic devices is converted into a visible optical information signal in an electronic display device. Such an electronic display device serves as an interfacing means between a user and the electronic devices.
Meanwhile, owing to developments in the semiconductor technology, recent electronic devices are generally driven by lower voltage and lower power, and have a slimmer size and a lighter weight. With such a trend, a flat panel type display device which is slimmer and lighter and requires lower driving voltage and power becomes in more demand and desirable.
An LCD device among the various types of flat panel display devices is much slimmer and lighter than any other display devices, and has a lower driving voltage and lower power consumption, and also has the displaying quality similar to that of CRT-type display devices. Therefore, LCD devices are widely used in various electronic equipments.
Recently, an LCD device for performing a bi-directional image display has been developed.
Specifically, a conventional LCD device for performing the bi-directional image display includes a backlight, a first LCD panel and a second LCD panel. The first LCD panel is disposed above (or below) the backlight, and the second LCD panel is disposed below (or above) the backlight.
In the conventional LCD device for performing the bi-directional image display, light radiated from a lamp(s) is divided into two groups of light. A first group of light is provided to the first LCD panel, and a second group of light is provided to the second LCD panel. The conventional LCD device only has the function of dividing the light radiated from the lamp(s), but does not have the function of regulating an amount of each of the two groups. It is thus desired that an LCD device can divide the light radiated from the lamp(s) into two groups and also can regulate the amount of each of the two groups.
An LCD panel, which is available for the LCD device capable of performing the bi-directional image display, may have a structure in which the LCD panel can display images in a transmission mode or a reflection mode according to an amount of external light. The LCD panel includes a first substrate, a second substrate, a liquid crystal layer interposed between the first and second substrates, and pixel electrodes. The pixel electrodes are formed on the first substrate, and each of the pixel electrodes has a transparent electrode region and a reflective electrode region. Light is transmitted through the transparent electrode region in the transmission mode, and is reflected by the reflective electrode region in the reflection mode. Accordingly, the LCD panel displays images by means of the transparent electrode region in the transmission mode, and displays images by means of the reflective electrode region in the reflection mode.
The conventional LCD device having the above structure has at least the following problems.
First, since a display area of the LCD device is divided into a transmission area used in the transmission mode and a reflection area used in the reflection mode, it is not effective in aspect of utilization of the display area.
Second, since the conventional LCD device has to employ the wide band ¼ wavelength phase difference plates covering an overall frequency band of the visible ray, as well as a first and a second polarizing plates attached on each of the first and second substrates, a manufacturing cost is elevated compared with a transmission type LCD device that displays images by means of a backlight disposed under the LCD panel.
Third, since the polarization characteristic in the transmission mode causes a light loss of 50%, there are drawbacks in that a light transmissivity decreases by 50% and a contrast ratio (C/R) is lowered.
Fourth, since Δnd (Δn: a value for representing optical anisotropy or refractive anisotropy; d: cell gap) of a liquid crystal layer is only 0.24 μm which is a half of Δnd (0.48 μm) of the conventional transmission type LCD device, the cell gap of the liquid crystal cell should be decreased to a level of 3 μm, and the Δn of the liquid crystal also should be decreased. Accordingly, there are problems in that the manufacturing process becomes difficult and degeneration in the reliability of the liquid crystal is caused.